This application claims the priority of German patent application 199 03 113.4, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an apparatus for condensing a drafted fiber strand, comprising a suction device having a stationary sliding surface, also comprising a suction slit located in the sliding surface which suction slit extends essentially in transport direction, as well as a circulating, air-permeable transport belt which glides with a contact surface over the sliding surface, and which covers the suction slit while transporting the fiber strand over the suction slit.
An apparatus of this type is, for example prior art in German patent 197 41 441 C1. In this apparatus, the suction slit takes the form of a so-called NACA nozzle, which is not described in any detail. The aim is that as a result of the turbulences caused in the airflow at the unavoidable edges, a self-cleaning effect is achieved.
The above mentioned publication does not deal with the risk that fiber fly and trash can settle between the stationary sliding surface and the moving contact surface of the transport belt. Trash occurs in that single fibers of the fiber strand to be condensed, in particular the shorter fibers, reach with their front end in the area of the suction slit through the perforation of the transport belt into the inside. These fibers project then partly into the suction slit until the suction slit has been passed over. At best, the relevant fibers can be cut off at the end of the suction slit and the cut-off end sucked off; at worst, the fiber end is only turned over, so that it is not sucked off, but rather is transported further with the transport belt and lies partly between the contact surface and the sliding surface. The fiber can tear at any time and settle, dust-like somewhere on the contact surface or the sliding surface, in particular in the case of sticky fibers containing "honey dew". This results in the condensing effect being different from spinning station to spinning station, so that varyingly condensed fibers strands are delivered at the nipping points.
Condensing a drafted fiber strand, namely in an area to which the spinning twist is not retroactive, serves the purpose of bundling the fiber strand in its cross section and making it less hairy. This results, after the spinning twist has been imparted, in a smoother yarn with a higher tensile strength. When the desired condensing effect does not occur at even only one spinning station due to an impaired condensing zone, then the so-called Moire effect appears in the subsequent woven yarn, which renders the product a reject. It must be ensured therefore, that the condensing effect at the individual condensing zones of the spinning stations is constantly maintained.
It is an object of the present invention to ensure that the perforation of the transport belt does not become clogged and that no deposits can form in the area between the contact surface and the stationary sliding surface.
This object has been achieved in accordance with the present invention in that the sliding surface is provided with at least one sharp edge extending transversely to the transport direction, which sharp edge scrapes the contact surface at least in the area where it covers the suction slit.
By means of the sharp edge, fibers, whose turned-in ends project into the suction slit, are cut off so that they can be sucked off. The basic principle behind the present invention is that, at an exact defined point, in particular at the end of the suction slit, the contact surface of the transport belt is scraped from below. An edge of this kind can engage basically at any chosen point of the contact surface of the transport belt, but it is however purposeful to have the end of the suction slit in the form of this edge, as at this point the above mentioned problems are at their worst. By means of an edge bordering the suction slit, a fiber piece projecting into the suction slit can be shaved off and subsequently sucked off. The shortened fiber remaining on the transport belt finds its way back to the fiber strand and is transported off with the fiber strand after leaving the nipping point.
The suction slit can be in the form of a window closed on all sides and having a surrounding edge. The cutting-like edge thus surrounds the entire window and acts on the contact surface of the transport belt like a shaving device. It cannot happen that at this point a fiber is not cut off and drawn possibly further through the perforation, which would make the matter worse.
Alternatively or in addition, an edge can be applied at a distance from the suction slit to a separate suction opening, which edge then extends preferably over the entire width of the transport belt. In this case, the contact surface is not only shaved in the area which covers the suction slit, but is also kept clean over the entire effective width. The effect can be improved when the edge extends transversely to the transport direction, as the cutting effect is increased hereby. The separate suction ensures that the cut-off fiber ends are immediately drawn off. The suction effect can be increased when the contact surface in the area of the separate suction opening is free of any support, that is when the contact surface of the transport belt is not disposed at this point on the sliding surface. If a suction nozzle is arranged in addition to the suction opening on the side of the transporting belt facing away from the contact surface, the perforated transport belt can be suctioned in the area of the edge from both sides.
The material of the sharp edge must, of course, be hard enough in order that it does not wear. If, because of the necessary sliding properties of the stationary sliding surface, the material does not meet this requirement, then it is purposeful to provide a suitable coating. If the coating is very thin, this will not impair the sharpness of the edge.
Instead of a coating, the sliding surface can be provided with a flexible stocking-like coat, which is provided with at least one recess for forming the at least one edge. The stocking-like coat must be made of sufficiently strong yet thin material. The recess is located for the purpose of the present invention in the area of the suction slit, which is again in the form of a window. The periphery of the recess is hereby advantageously somewhat smaller than the periphery of the suction slit, so that the suction slit is bordered on all sides by a sharp edge.
Alternatively the sliding surface can be surrounded by a thin metal foil, which is provided with at least one recess for forming the at least one edge. This metal foil can be adhered or clipped on. It should have a thickness of between 0.2 mm and 0.5 mm, so that the edge remains sufficiently sharp. The recess is so chosen that it is disposed on the suction slit, but reduces, window-like, the size of the slit all around. In particular the end of the suction slit should be reduced in size by the metal foil, as the sharp edge is particularly effective at this point.
The metal foil can have open impact points, which are located in the area of a concave dent in the sliding surface. The metal foil can be made as an open metal strip, which corresponds to the periphery of the stationary sliding surface. At the dented area, the metal foil is raised from the contact surface somewhat so that the transport belt is not subject at this point to any significant friction. At such a concave dent, the impact points of the metal foil are not damaging.